Pathological features in lesions at an early stage of atherosclerosis involve the event of increase of foam cells in artery walls. Scavenger receptors (hereinafter abbreviated as “SR”) that are present on a cell membrane of a macrophage (Krieger, M. et al., Annu. Rev. Biochem., 63, 601-637, 1994) lack negative feed back regulation by cholesterol, alien from LDL receptors. Thus, the receptor itself changes into foam cells through actively incorporating modified LDL (low density lipoprotein that is a complex of cholesterol and a lipoprotein) to accumulate beneath the vascular endothelial cells. Therefore, macrophages and SRs thereof have been believed to play important roles in the establishment of pathosis of atherosclerosis (Brown, M. S. et al., Nature, 343, 508-509, 1990; Kurihara, Y. A. et al., Current Opinion in Lipidology, 2, 295-300, 1991; Krieger, M., TIBS, 17, 141-146, 1992; Krieger, M. et al., J. Biol. Chem., 268(7), 4569-4572, 1993).
Continuous hyperglycemia in a living body resulting from diabetes causes nonenzymatic glycation of various proteins, thereby leading the production of Maillard reaction-advanced end products (AGE: advanced glycation end products), which are final products in a glycation process via a Schiff base and an Amadori compound. AGE having an injurious action on cells adversely affects through the binding to macrophages, vascular endothelial cells, hepatic cells, renal mesangium cells and the like via AGE receptors. For example, it is known that secretion of cytokines such as TNF (Tumor Necrosis Factor), IL-1 (Interleukine-1) and platelet derived growth factor (PDGF) is accelerated upon binding of AGE to a macrophage, thereby causing cell injuries characteristic to diabetic complications. SR is believed to participate profoundly in diabetic complications such as diabetic nephropathy, diabetic retinopathy, diabetic neuropathy, on the basis of the findings that SR is one of the receptors involving in incorporation and degradation of AGE (Araki, N. et al., Eur. J. Biochem., 230, 408-415, 1995; Suzuki, H. et al., Circulation, 92, I-428, 1995), and that degradative activity of AGE is lowered to a level of third in an SR-double knockout mouse. Further, when an excessive AGE albumin is administered to a rat, AGE was found to deposit in kidney, thereby developing and glomerulosclerosis (Vlassara, H. et al., Proc. Natl. Acad. Sci. USA, 91, 11704-11708, 1994). Accordingly, SR, which recognizes AGE, is anticipated to profoundly involve in glomerulosclerosis.
In addition, SR is believed to involve in Alzheimer's disease. Pathological features of Alzheimer's disease concern senile plaques that are deposits of β-amyloid. β-amyloid has been reported to activate microglia cells via SRs that are expressed on the microglia cells to generate active oxygen, leading to the expression of neurotoxicity (Nature, 382, 716-719, 1996).
Examples of ligand for SRs include: ligands having negative charge, e.g., modified LDL such as acetylated LDL (AcLDL), oxidized LDL (OxLDL) and the like, modified proteins such as maleylated BSA and the like, quadruple helical nucleic acids such as polyinosinic acids and the like, polysaccharides such as dextran sulfate and fucoidane and the like, acidic phospholipids such as phosphatidylserine, phosphatidylinositol and the like, endotoxin (LPS), AGE, senile cells apoptotic cells, and the like, although differences in specificity thereof may exist depending on the differences of molecular species of SRs. Additionally, SR is believed to play an important role in removal of foreign substances, metabolic decomposition products and the like, because SR extensively recognizes various modified substances and a wide variety of foreign substances such as viruses in a living body (Hampton, R. Y. et al., Nature, 352, 342-344, 1991; Tokuda, H. et al., Biochem. Biophys. Res. Commun., 196(1), 8-24, 1993; Pearson, A. M. et al., J. Biol. Chem., 268, 3546-3554, 1993; Dunne, D. W. et al., Proc. Natl. Acad. Sci. USA, 91, 1863-1867, 1994; Freeman, M. W.; Current Opinion in Lipidology, 5, 143-148, 1994).
SRs have been expressed in hepatic sinusoidal endothelial cells (Eskild, W. et al., Elsevier Biomedical N.Y., 255-262, 1982), vascular endothelial cells (Baker, D. P. et al., Arteriosclerosis, 4, 248-255, 1984; Bickel, P. E. et al., J. Clin. Invest 90, 1450-1457, 1992), blood smooth muscle cells (Pitas, R. E. et al., J. Biol. Chem., 265, 12722-12727, 1990; Bickel, P. E. et al., J. Clin. Invest., 90, 1450-1457, 1992), fibroblasts (Pitas, R. E. et al., J. Biol. Chem., 265, 12722-12727, 1990), and the like as well as in macrophages. Further, SRs have been classified into SRA, SRB, SRC (Peason, A. et al., Proc. Natl. Acad. Sci. USA, 92, 4056-4060, 1995), FcγRIIB2 (Stanton. L. W. et al., J. Biol. Chem., 270, 22446-22451, 1992) and macrosialin (CD68) (Ramprasad, M. P. et al. Proc. Natl. Acad. Sci. USA, 92, 9580-9584, 1995), human vascular endothelial OxLDL receptor (LOX-1: lectin-like oxidized LDL receptor) (Sawamura, T. et al., Nature, 386, 73, 1997). Moreover, SRA has been classified into SR-AI and SR-AII (Kodama, T. et al., Nature, 343, 531-535, 1990), and MARCO (a novel macrophage receptor with collagenous structure) (Elomaa, O. et al., Cell, 80, 603-609, 1995); SRB has been classified into CD36 (Endemann, G. et al., J. Biol. Chem., 268, 11811-11816, 1993) and SR-BI (Acton, S. L. et al., J. Biol. Chem., 269, 21003-21009, 1994).
SR-AI and SR-AII are homotrimers, which are of inside-out type transmembrane proteins of which N-terminus resides within the cell. The protein is structurally revealed to have several domains such as a collagen-like domain, α-helical coiled coil domain and a cysteine-rich domain, and the like in its extracellular portion (Rohrer, L. et al., Nature, 343, 570, 1990; Matsumoto, A. et al., Proc. Natl. Acad. Sci. USA, 87, 9133, 1990). The collagen-like domain has a structure characteristic in collagen, (Gly-Xaa-Yaa)n, wherein Xaa and Yaa may be any one of amino acid residues, and this domain functions as a ligand-binding site. The α-helical coiled coil domain is a dexiotropic hepted repeat which turns two times at every seven amino acids, namely having a structure of α-helical coiled coil. The three polypeptides form a homotrimer with hydrophobic amino acids such as leucine and isoleucine that are present at every seven amino acids being directed to inside of the molecule, whilst having polar amino acids and carbohydrate chain-binding site outside thereof (leucine zipper). Roles of the domains involve retention of the homotrimer structure, as well as binding to the ligands such as modified LDL to incorporate them into the cells, and changing a tertiary structure of the receptor depending on decrease of pH in endosome, finally resulting in the dissociation of the ligands.
Intracellular domain of the protein has a tight turn structure, which is characteristically found in an endocytotic signal, similarly to the structures including NPXY sequence found in LDL receptors or insulin receptors and YXRF sequence found in transferrin receptors. It is suggested that endocytosis may be suppressed when these sequences are deleted.
SR-A1 and SR-A2 arise from alternative splicing of mRNA coding a cysteine-rich domain. SR-AI has 110 amino acids corresponding to the domain, whilst SR-AII has corresponding 17 amino acids. SR-AI and SR-AII are expressed in at least peripheral macrophage derived from monocyte, pulmonary alveolus macrophage and hepatic Kupffer cell. It is revealed that they participate in a host defense system in a living body, for example, arteriosclerosis, Calcium ion-independent cell adhesion and the like (Krieger, M. et al., Annu. Rev. Biochem., 63, 601-637, 1994; Wada, Y et al., Ann. N.Y. Acad. Sci., 748, 226-239, 1995; Fraser, I. P. et al., Nature, 364, 343, 1993). Further, OxLDL is present within macrophages of arteriosclerosis foci. In addition, SR-AI and SR-AII are abundantly expressed on the cell membrane of macrophage, and the elevation of blood lipoprotein by lipid absorption is suppressed in a transgenic mouse for SR-AI. Accordingly, it is envisaged that SR-AI and SR-AII play important roles in incorporation of Ox LDL.
To the contrary, although MACRO classified into SRA has a similar structure as that of SR-AI, it has no α-helical coiled coil domain, which is characterized by having a long collagen-like domain. MACRO is expressed in spleen macrophage, lymph node macrophage and the like, which is believed to function in a host defense mechanism against bacterial infection in a living body taking into account of the specificity of the ligands thereof.
Suzuki et al., successfully produced an SRA-knockout mouse through the substitution of the fourth exon that is a common part between SR-AI and SR-AII with a neomycin resistant gene (Suzuki. H. et al., Nature, 386, 292-296, 1997). Immune disorder has been observed in the SRA-knock out mouse in comparison with the wild type, and exhibits a high rate of infection with Listeria and herpes simplex virus. In addition, it is indicated that SRA participates in phagocytosis of T cells having apoptosis occurred, and that the phagocytic capacity is reduced in the SRA-knockout mouse in comparison with the wild type (Platt, N. et al., Proc. Natl. Acad. Sci. USA, 93, 12456, 1996). Furthermore, in a double knockout mouse obtained by the mating of the SRA-knock out mouse and an apoE deficient mouse (Plump, A. S. et al., Cell, 71, 343, 1992; Zhag, S. H. et al., J. Clin. Invest., 94, 937, 1994) that is an animal model for arteriosclerosis, it is indicated that the area of arteriosclerosis foci is significantly smaller than that of the apoE deficient mouse (Suzuki, H. et al., Nature, 386, 292-296, 1997).
Thus, SR can be utilized in the elucidation of functions of macrophage, the elucidation of mechanisms of development of various types of diseases including for example, arteriosclerosis, diabetic complications and AD, hyper β-lipoproteinemia, hypercholesterolemia, hypertriglyceridemia, hypo α-lipoproteinemia, transplantation, atherectomy, post angiogenic restenosis and the like, as well as diagnostic, prophylactic, therapeutic methods thereof, and in the development of reagents and drugs for the same. Accordingly, to find novel molecular species belonging to this family can be the means to solve the above-described problem to be solved.
Besides, a complement system that plays an important role in a host defense mechanism is known to include: a classical pathway in which an immunoglobulin serves as a recognition molecule followed by the activation of C1 that is the first component of the complement; and an alternative pathway in which C3, which is the third component of the complement, is directly coupled to foreign substances such as bacteria. In addition to these pathways of the complement activation, a lectin pathway was illustrated in which a mannose binding protein (hereinafter referred to as “MBP”), which is a serum lectin, activates the complement system through the direct recognition of and coupling with a carbohydrate chain on the surface of the foreign substance, in recent years (Sato, T. et al., Int. Immunol., 6, 665-669, 1994).
MBP is a C type lectin which specifically binds to mannose, N-acetylglucosamine and the like in the presence of Calcium ion, of which structure comprises a collagen-like domain containing at least (Gly-Xaa-Yaa)n, and carbohydrate recognition domain (CRD). Similarly to MBP, lectins having a collagen-like domain and CRD are generically called as collectin (Malhotora, R. et al., Eur. J. Immunol., 22, 1437-1445, 1992), which include collectin-43 (CL-43), surfactant protein A (SP-A), surfactant protein D (SP-D), bovine conglutinin (BKg) and the like, in addition to MBP. Collectin has an opsonic activity, which is believed to participate in basal immunity against a variety of microorganisms such as bacteria and viruses (Kawasaki, N. et al., J. Biochem., 106, 483-489, 1989; Ikeda, K. et al., J. Biol. Chem., 262, 7451-7454, 1987; Ohta, M. et al., J. Biol. Chem., 265, 1980-1984, 1990; Summerfield, J. A. et al., Lancet, 345, 886, 1995).
These collectins are known to constitute from a basic structure containing characteristic domains such as (1) CRD and (2) collagen-like domain and the like as shown in FIG. 1(a) (Malhortra et al., Eur. Immunol., 22, 1437-1445, 1992). This basic structure forms a subunit through composing a triple helix at the collagen-like domain, and thus these subunits further form an oligomer structure such as trimer, tetramer, hexamer and the like.
Recently, collectins were suggested to participate in non-specific immune response, e.g., it was reported that for example, they are playing important roles in neutralizing and excluding various microorganisms in infants having maternal antibodies from the mother or nonspecific defense systems which were insufficiently developed (Super et al., Lancet, 2, 1236-1239, 1989). Moreover, results of investigation are reported involving in roles of these collectins in the body defense system of a host, which for example, suggest that the host becomes more susceptible to infections through the lowered concentration of MBP in blood resulting from genetic mutation of MBP (Super et al., Lancet, 337, 1569-1570, 1991). In addition, it was reported that scrum MBP content shows a lowered level upon the failure of opsonization (Madsen, H. O. et al., Immuno genetics, 40, 37-44, 1994), whilst bacterial infections readily occur (Garred, P. et al., Lancet, 346, 941-943, 1995). Therefore, MBP may be believed to play important roles in an immune system.
The present inventors previously found that BKg and MBP inhibit infections by H1 and H2 types influenza A viruses as well as a hemagglutination activity (Wakamiya et al., Glycoconjugate J., 8, 235, 1991; Wakamiya et al., Biochem. Biophys. Res. Comm., 187, 1270-1278, 1992). Thereafter, a cDNA clone encoding BKg was obtained, and the relevance between BKg and SP-D and the like has been also found (Suzuki et al., Biochem. Biophys. Res. Comm., 191, 335-342, 1993).
Likewise, collectins are substances to which usefulness in the elucidation of host defense mechanism and utilities as a biologically active substance are expected. Thus, the finding of novel molecular species belonging to this family may greatly contribute in various medical fields and biological fields in addition to the therapy of infectious diseases.